From the Game Website
The Legend of Zelda: Ocarina of Time Master Quest is a special version of Ocarina of Time that wasn't released for the N64 in North America. The game features more challenging dungeons, tougher puzzles, and the familiar Ocarina story.

The Legend of Zelda: Ocarina of Time, considered by many to be the best adventure ever created, arrives on the Nintendo Gamecube, faithfully reproduced and displayed in high resolution. As a special bonus, this disc also contains the never-before-released Master Quest, a second journey through Ocarina of Time with newly laid-out -- and incredibly challenging -- dungeons. Only true heroes will make it through this quest alive...